The present invention relates generally to a suspension control system for an automotive vehicle with variable damping characteristics of shock absorbing characteristics depending upon a vehicle speed. More specifically, the invention relates to a suspension control system which includes a sensor monitoring road surface conditions for use in controlling the stiffness of the suspension in accordance with roughness of the road surface, and which is variable of sensitivity of the road condition sensor or damping characteristics depending on the vehicle speed.
Various uses of road preview sensors have been proposed and developed. For example, SAE Technical Paper Ser. Nos. 630,750 an 800,520, respectively published on October, 1968 and February, 1980 show road preview sensors for use in suspension systems for obtaining optimum riding comfort and drivability. In addition, Japanese Patent First Publication No. 57-172808, published on Oct. 23, 1982 discloses a vehicle height control system which includes a sensor which detects rough road conditions and adjusts the vehicle height level depending upon road surface conditions. A vehicle height or level sensor is employed in the disclosed vehicle height control system for monitoring the relative displacement between the vehicle body and wheel axle. The output of the vehicle level sensor is compared with a reference level, which serves as a rough road criterion, and adjusts the vehicle height according to the result of judgement of the road surface conditions.
In another example, Japanese Patent First Publication No. 58-30542, published on Feb. 23, 1983, discloses a variable damping force shock absorber with damping characteristics varying in accordance with vehicle driving conditions. In the disclosed system the magnitude of relative displacement between the vehicle body and wheel axle is measured and a vehicle height variation indicative signal is derived from the measured displacement and the instantaneous vehicle speed. The vehicle height variation indicative signal value is compared with a reference value which serves as a staff suspension criterion for adjustment of the damping characteristics ofthe shock absorber in accordance therewith.
Such road sensors or preview sensors are effective for detecting road surface condition and controlling damping characteristics of the vehicular suspension for providing both of riding comfort and driving stability. On the other hand, as is well known, when the vehicle is driven in substantially low speed, pitching motion of the vehicle significantly degrades riding comfort. Likewise, when the vehicle speed is substantially high, harshness level of the suspension increases due to vibration of the road wheels if the damping force is not enough. Therefore, in such speed range, harder suspension is preferable for preventing pitching motion in substantially low vehicle speed and reducing harshness level in substantially high vehicle speed. Alternatively, when the vehicle speed is inbetween the substantially low and high speeds and thus within the normal speed range, e.g. 30 km/h to 80 km/h, hard suspension will give rough ride filing to degrade driving comfort. Whereas the conventional suspension control system employing the road sensor for controlling suspension characteristics depending on the road surface condition, can not adjust the sensitivity of the road sensor or damping characteristics depending on the vehicle speed. Therefore, the conventional systems have not been completely satisfactory in providing both of riding comfort and driving stability.